Use of hydrogen as a new energy source has come to attention in recent years. Due to safety concerns and poor public perceptions about hydrogen, one of the most important issues in advancing the hydrogen-based industries is development of highly reliable hydrogen detection techniques.
Conventionally used hydrogen detection means are mostly catalytic combustion type and semiconductor type. The former causes a heated thin line of platinum or palladium to react with hydrogen to detect variations in electric conductivity; the latter provides an electrode pair in a layer of an oxide semiconductor such as tin oxide on a substrate to detect an increase in the number of carriers in the semiconductor layer due to contact with hydrogen. However, these approaches have a drawback that they entail the danger of ignition because there is an electric contact in a sensor part, thereby requiring explosion protection means.
On the other hand, several approaches to detecting hydrogen by using an all-optical sensor part have been studied. The approaches do not have the drawback described above and have the advantage of a high level of safety.
For example, Patent literature 1 describes a technique in which a hydrogen absorbing film is formed in a constricted portion of an optical fiber and expansion of the volume of the hydrogen absorbing film changes bending loss that occurs in the constricted portion of the optical fiber. FIGS. 1A and 1B illustrates a configuration of a sensor part of a hydrogen sensor described in Patent literature 1. In the figures, 11 denotes an optical fiber, 12 denotes a constricted portion of the optical fiber 11 which is a narrowed portion of the optical fiber 11, 13 denotes a hydrogen absorbing film, and 14 denotes a reflecting mirror which reflects measurement light that entered the optical fiber 11. In this example, a change in the intensity of reflected light is detected to detect hydrogen.
Patent literature 2 describes a technique which uses a hydrogen sensitive switchable mirror to detect a change in light reflectance and transmittance. FIG. 2 illustrates a configuration of a hydrogen sensor described in Patent literature 2. The hydrogen sensor includes a probe 22 with a hydrogen sensitive switchable mirror 21 disposed to be exposed to an atmosphere of interest 27, a light source 23 which emits probe light 25 toward the probe 22, a photodetector 24 which receives detection light 26 reflected by the probe 22, and an optical waveguide 29 having one end disposed close to the probe 22 and the other end connected to the light source 23 and the photodetector 24 through an isolator 28. In this example, the intensity of the detection light 26 can be monitored with the photodetector 24 to detect presence or absence of hydrogen in the atmosphere of interest 27 and the concentration of hydrogen in the atmosphere of interest 27.
On the other hand, Non-patent literature 1 describes a technique which uses a surface plasmon resonator configured with periodic holes formed in a thin film of palladium, which is a hydrogen absorbing metal, to detect a decrease in the amount of transmitted light due to absorption of hydrogen by the thin film.